Heat treatment furnaces where the heating unit is located inside of a vacuum chamber are provided with special electrical lead-through or feed-through devices, which serve for transmitting the current to the heating elements in the vacuum chamber. These lead-through devices must satisfy severe requirements due to extreme operational circumstances, as for instance high current, high temperatures, sealing tightness, vacuum retention and durability when exposed to high pressure (for instance in pressure sintering furnaces).
In the vicinity of such a lead-through device, an electrical breakdown of the insulating gap path can occur due to mechanical irregularities, as for instance flaws in the insulation ceramics and contamination of the socket or stud passage or due to overvoltage. This can result in a gaseous discharge which is initiated by a high-frequency spark that, however, is present for a time span of only several milliseconds and then continues to exist as an electric arc.
A transfer of material occurs during the gas discharge in the form of an electric arc. Because of this, on the one hand, components between which the gas discharge occurs are weakened by removal of material so that fracture can occur, which causes partial destruction of the lead-through device. On the other hand, the removed material and possible fragments of parts of the lead-through device or their environment constitute a danger for the entire installation as well as a disturbance factor for the method to be performed.
Since such a gas discharge cannot be prevented in spite of all measures to the contrary, it would be greatly advantageous to at least be able to detect same. Detection with the furnace closed in the course of a process is however very difficult. High frequency monitoring is problematic because the high frequency spark is present only for a time span of several milliseconds or less. Thereupon the discharge converts into an electric arc, which cannot be detected by means of high frequency. A detection by insulation resistance in the course of this phase can only be performed with great difficulty, since the insulation resistance between the heating device and other installations, which partially consist of a graphite mat, has a resistance of only a few ohms and since, in addition, the heating voltage is always superimposed on the measurement.